Brønsted acidity of Al/SBA-15

To increase the content of well-dispersed aluminum in Al/SBA-15 and its Brønsted acidity, a water-free synthesis method involving a Si–O–Al precursor was employed to prepare a series of Al/SBA-15 of widely varying composition. The materials were characterized with ICP, BET, TEM, 27Al MAS NMR and IR spectroscopy and compared to conventionally prepared Al/SBA-15. For low Al content (Si/Al > 10), the conventional approach leads to highly dispersed, predominantly isolated Al with well-ordered mesopores. At higher Al content, the order decreased or was even completely lost. In general, this led to the agglomeration of Al species on the surface. Although in most cases a substantial part of Al atoms exhibit tetrahedral coordination indicating their high dispersion and interaction with the silica, the overall Brønsted acidity of these Al/SBA-15-type materials is very low. The overall acidity of all Al/SBA-15 is of the same order as amorphous silica–alumina. By IR spectroscopy of H/D exchange with perdeuterobenzene, it was found that the intrinsic acidity of the acid sites in Al/SBA-15 is similar to that in zeolites and amorphous silica–alumina, thus further confirming that acid site concentration dominates differences in acidity of aluminosilicas.

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► SBA-15 was prepared in a wide range of Si/Al ratios (1–60).
► Below Si/Al = 10 the ordered mesoporous structure is nearly completely lost.
► Nearly all Al can be forced to be present as tetrahedral Al for not too high Al content.
► Only a very small fraction of tetrahedral Al contributes to Brønsted acidity.
► The Brønsted acidity of SBA-15 is very similar to that of amorphous silica–aluminas.